1. Field
The present invention relates to a microcomputer which obtains a rotor position signal from a biphase signal delivered from a resolver mounted on an electric motor, and a motor control system provided with the microcomputer.
2. Related Art
Low-cost position sensitive detectors such as a Hall element need to be disposed at a predetermined location of an electric motor to detect a rotor position in order that a brushless DC motor may be driven. However, an angle detected by the Hall element and indicative of a rotor position is coarse such that a position accuracy is low. In particular, when the Hall element is used for sinusoidal drive which necessitates a high accurate position detection, a detected angle is interpolated by a microcomputer or the like.
However, when the microcomputer is used as a position detecting unit, a ratio of position detection processing to a processing load of CPU is increased particularly in the use with electric motors driven at high speeds. This results in a problem that a time when the motor can be controlled is limited except for a period of execution of a position detection processing. In view of the problem, the use of a resolver which can detect a rotor location with high accuracy in a hardware manner has recently been increased.
A resolver signal needs to be converted to data in a motor system using a resolver so as to be loaded into a control element of the microcomputer or the like. A dedicated signal converter has been used in many cases for this purpose. For example, Japanese patent application publications JP-A-2005-114442 and JP-A-H11-83544 disclose arrangement examples of resolver data converters (RDCs) respectively. FIG. 5 schematically illustrates a motor control system using an RDC. A resolver 2 is mounted on a brushless DC motor 1 and includes an excitation coil to which an oscillator serving as data converting IC and incorporated in RDC 3 supplies an excitation signal. Output signals delivered from a cosine coil and a sine coil respectively are supplied into RDC 3. The output signals are differentially amplified. RDC 3 converts a rotor position of the motor 1 to, for example, about 12-bit digital signals based on the cosine and sine signals (biphase signals) delivered from the resolver 2, thereby delivering the digital signals. When reading the digital signals delivered from RDC 3 from an input port thereby to obtain information about rotational position of the motor 1, CPU 5 of a microcomputer 4 controls the motor 1 via an inverter 6 based on the position information.
In Japanese patent application publication JP-A-2005-114442, the converted data is transferred to the microcomputer by serial communication. For example, when data delivered from the resolver has a resolution of about 12-bit as described above, serial transfer of the data necessitates the time of 6 μs even with the use of a 20-MHz clock. Position accuracy information goes short even in the case of data renewal at intervals of 6 μs when a motor is driven at high speeds. Accordingly, data needs to be corrected at the microcomputer side.
In order that the aforementioned time delay may be eliminated, it is contemplated that data be parallel transferred to the microcomputer, as proposed by Japanese patent application publication JP-A-H11-83544. In this case, however, a large number of terminals of the microcomputer are used exclusively for processing resolver signals and accordingly, the number of terminals which can be used as I/O, whereupon a processing capacity of the microcomputer as a control system is decreased. To overcome the disadvantage, the assignee of the present application filed a patent application in Japan and the application was published under the publication number JP-A-2005-210839 on Aug. 4, 2005. In the Japanese patent application, the assignee has proposed an arrangement that resolver signals are directly delivered to a microcomputer to be processed by the use of software.
According to the technique disclosed by JP-A-2005-210839, a microcomputer which obtains a rotor position signal from a biphase signal delivered by a resolver mounted on an electric motor according to a rotation phase of a rotor relative to a stator, thereby controlling the motor based on the rotor position signal, the microcomputer comprising a digital signal converter which is configured by hardware and converts the biphase signal delivered from the resolver into a digital data position signal, the digital signal converter being mounted on a single chip, the terminals of the microcomputer are used only for input of resolver signals. Accordingly, obtained positional information can be transferred to CPU without delay. However, a module for executing conversion processing needs to be incorporated into a control program, and the processing capacity of the microcomputer is accordingly reduced as described above. Thus, although processing in a transmission system of the resolver signals can be sped up, the processing capacity of the microcomputer sometimes imposes restrictions on the high-speed control.